This invention relates to the refrigeration portion of an air cycle environmental control system for an aircraft cabin.
Environmental control systems ("ECS") for aircraft are known to have a refrigeration portion that employs either a two, three or four wheel air cycle machine ("ACM"). The ECS is used to cool, filter, dehumidify, pressurize and otherwise condition the air supplied to the cabin. In most installations, the ECS utilizes a source of hot compressed air, such as from the engine compressor section (during aircraft flight with the engines operating), the auxiliary power unit ("APU") (during ground operation), or both. The hot pressurized-bleed air is then expanded and cooled in the ACM to provide a cool, fresh air supply to the cabin. Numerous schemes for accomplishing this result are known in the art. See, for example, U.S. Pat. Nos. 2,767,561; 4,265,397; 4,334,411; 4,374,469; and 5,086,622.
Most modern environmental control systems use ram cooling air to initially cool the relatively hot, pressurized bleed air before the bleed air is presented to the ACM. The ram air is vented from the ambient air rushing by the moving aircraft. However, ram air usage presents a relatively small, yet significant amount (approximately 21/2%) of the total aerodynamic drag or resistance of the aircraft. Increased drag translates into increased fuel consumption. Thus, the aerodynamic drag associated with the use of ram air for cooling purposes is a major ECS operating penalty.
It is also known in the art to provide a three wheel ACM for use on aircraft that utilizes a high-pressure water separation concept to deliver a sub-freezing air supply. Therein, condensate is removed from the bleed air upstream of a cooling turbine. The warm bleed air is cooled in the condenser to a temperature below its dewpoint by the cooler turbine exhaust air. After the water condensate is separated out and removed from the bleed air, the resulting dehumidified air is reheated prior to entering the cooling turbine. Reheaters restore much of the thermodynamic efficiency loss caused by the separation process. This high-pressure water separation concept can supply air close to 0.degree. F., thereby reducing the airflow required to cool the cabin, further saving on engine fuel consumption.
However, this sub-freezing, three wheel ACM refrigeration system is relatively inefficient thermodynamically when compared to a more modern four wheel ACM described in U.S. Pat. No. 5,309,735. Also, means must be provided for the sub-freezing system to prevent the accumulation of ice in the ducting due to the freezing of entrained moisture in the bleed air. As stated in U.S. Pat. No. 4,334.411, not all of the entrained moisture can possibly be removed from the bleed air. Therefore, warm air (i.e., air above the freezing point of water) must be introduced at various points into the system. The '411 patent illustrates one scheme for introducing warm air into a system employing a turbine that expands and cools air to a temperature below the freezing point of any water contained in the air.
Accordingly, it is a primary object of the present invention to provide an aircraft environmental control system that reduces the amount of ram air used to cool the engine bleed air.
It is a general object of the present invention to reduce ram air usage, thereby reducing aerodynamic drag of the aircraft.
Another object of the present invention is to eliminate the usage of sub-freezing heat exchangers found in prior art three wheel ACM designs, thereby reducing the weight of the main heat exchangers.
It is a further object of the present invention to provide an environmental control system that is significantly more fuel efficient than prior art designs.
It is another object of the present invention to expand the cabin exhaust air in a turbine portion of the ACM to increase the power provided to a compressor rotor portion of the ACM, therein reducing the needed bleed air pressure for a required cabin air flow, the reduced amount of pressure for the bleed air translating into a more fuel efficient system.
The above and other objects and advantages of the present invention will become more readily apparent when the following description is read in conjunction with the accompanying drawings.